Cryotreatment, particularly cryoablation, is frequently used to treat a variety of cardiac arrhythmias, including atrial fibrillation. Many cardiac arrhythmias are caused by or involve the conduction of aberrant electrical currents through cardiac tissue. During cryoablation, tissue is cooled by a cryotreatment catheter until the aberrant electrical conduction is eliminated or otherwise deemed to be ameliorated. For example, some aberrant electrical currents may originate from within one or more pulmonary veins. In this case, the pulmonary vein ostium and/or antrum may be cooled until the pulmonary vein(s) is/are isolated from the left atrium or other cardiac tissue.
The time to effect (TTE) may be defined as the time it takes for an area of cardiac tissue to stop conducting electricity. This is known to occur when all the conducting tissue around an ablation site has reached a temperature below approximately +23° C., at which temperature the cells do not repolarize, and it can happen only once the deepest conducting fiber of tissue has reached that temperature. Therefore, a longer time to achieve TTE indicates the presence of fibers that are more difficult to cool, and this also correlates to a longer time to achieve permanent isolation (TTI). Both TTE and TTI may be correlated to each other according to the thickness of the tissue to be ablated (or to be more precise, where the deepest conducting fiber is located), such as the thickness of a target pulmonary vein, and/or according to the quality of ablation (such as surface contact, push force, alignment of the cryotreatment element with the tissue, quality of pulmonary vein occlusion, or the like). So, better contact quality and a thinner area of tissue results in a shorter time to achieve TTE and, therefore, the smaller the dose of thermal treatment.
When treating particular regions of tissue, it may be difficult to direct or control the depth of the cryoablation. For example, the reduction in tissue temperature may not be contained to the exact region or depth desired for treatment using currently known methods, and this often necessitates having to carefully monitor lesion formation and/or tissue temperature in real time, which can be difficult and imprecise, and/or having to assess lesion formation after a cryoablation procedure, which can result in unintended damage if not monitored during the procedure. Collateral damage to non-target tissue may result if a lesion extends too far or too deep beyond the target treatment area. Conversely, the procedure may not correct the aberrant electrical conduction if the treatment is not delivered for an adequate amount of time or at a sufficient tissue depth.
It is therefore desired to provide a system and method for predicting or determining the optimum dose of cryotreatment to an area of target tissue to achieve isolation based on the time to effect (TTE).